Conclusiones parciales

The dedication and care in the configuration of the process require not only the knowledge of the device but also the details of the steps to follow. It is also necessary to understand the chemistry of the resin and the time allowed to process it. Increasing the temperature decreases the molding time, but it becomes difficult to maintain a uniform temperature across the device, this is because not all the heaters respond at the same speed. By decreasing the molding time, an impact on the decision-making and concentration on the operator was observed. Consider that he must follow the process program and at the same time he has to remove the gloves to adjust the temperature controllers and put them on again to handle the hot parts. This represents a latent danger. In this sense, we believe that the device requires at least two well-coordinated people to handle it. However, a computer-aided process will provide the basic tools to coordinate the machine, record the data and provide the operator with warnings or visual cues that contribute to the decision-making.

Continuing with the observations, we found that prior to the mixing, the resin has a consistency like the one of glycerine. As we continue stirring the mixture and the components are integrated, the viscosity decreases obtaining a consistency of corn syrup. If under-mixing is presented, the final part becomes fragile due to uncured spots where the resin was not catalyzed properly. In this context, the consistency of the liquid is a good indicator of the mixture status.

Regarding to the injection, the combination of compressed air and glass containers is very convenient since they allow to keep the injection pot clean and the device safe from the resin. Also, they are easily replaced or cleaned with a solvent. However, a minimum pressure is necessary to push the resin out of the injection pot. Due to the increase in temperature the resin is very watery which causes it to flow very quickly. A sudden increase in the pressure can cause the resin to flow from one end to the other, leaving the mold completely empty. In other words, the pressure range is very limited and difficult to control with the regulating valves and the operator could miss the opportunity to close the outlet port of the mold. To this purpose, the resin volume increases to allow the material to flow after filling the mold in a process called bleeding. The idea is to allow the resin to fully impregnate the fiber, remove air pockets and give time to the operator to close the pipes.

As a result, the amount of resin to inject increase, and in the same way the waste.

In general, the temperature controllers reach the desired temperature, but we had to adjust the set point as many times as required to obtain a close value. For instance, if the required temperature was 140 °C, at the end of the cycle the set point was between 3 and 8 °C more than that value.

Despite fine-tuning the issue persisted, it may be necessary to increase the heat capacity of the heaters or add proper isolation. As a matter of fact, these upgrades will improve the controller capabilities and increase the actual maximum temperature ramp of 6 °C/min. Although it is necessary to consider the amount of mass (mold + sample) to be heated, which can hinder to reach higher rates of heating.

Finally, removing the sample from the mold is extremely difficult, the release agent helps to this purpose but results not effective for the sides of the mold. The problem is that upon completion many parts of the device end up filled with polymerized resin. As result, tubes, quick connectors, ball valves must be replaced. The mold filling ports are not an exception and the resin from a pair of polymer pins that holds the molded part in place. Figure 3.9 shows the part with the pins that

must be broken with a rubber mallet to free the molded piece. Inevitably, the impacts could also break the piece and the filling holes needs to be re-drilled after each process.

Figure 3.9: Sketch of a molded material with the solidified filling ports attached

Table 3.1 summarizes the manufacturing and design challenges obtained from the industrial experience we carried out. They were also dissociated from the RTM laboratory scale device limitations and grouped into typical manufacturing evaluation criteria.

Table 3.1: Mechatronic device design challenges

Challenge Evaluation

Criteria Relevance to the project

Ensure resin mixture

Quality It is under the user responsibility but adding a mixer will increase the success rate.

Functionality Variation on the viscosity and injection flow. Heating and injection force must compensate.

Production

A batch of resin for a series of experiments could be made to maintain the mix uniform but requires storage at negative temperatures to decrease the chemical reaction rate. The reaction does not stop.

Resin stickiness and solidification outside the cavity mold

Maintenance

Plastic tubes and containers should be replaced. Valves, quick connectors can be cleaned but the solvents will damage the sealing. The injection reservoir must be empty.

Functionality Solid resin inside the tubes could block the injection.

Production The ports in the mold will solidify and should break at unmold without damaging the part.

Environmental Strong solvents required to clean the resin are being banned in some regions due to their toxicity.

Table 3.1 (continued): Mechatronic device design challenges Challenge Evaluation

Criteria Relevance to the project

Open and close device ports

Functionality

Ball valves are easy to handle and perform well blocking the flow, but they must be replaced too often.

Clamping the tube is other solution but the tube could deform or damage. Also, air compressed clamps exist but they are bulky.

Ergonomics

Synchronization and selection of the resin path is a key aspect of the process. It should not require of two people to work with the device.

Safety Handling hot surfaces always implies a burning hazard.

Maintain and

uniform temperature across the device

Functionality

Thermal shock could suddenly solidify or degrade the resin. Tubes and valves represent the major challenge for reaching this objective. A good number of temperature sensors may be required. Also, high power heaters and good isolation of the parts contribute to the temperature uniformity.

Ergonomics Heaters, power and sensor cables could be on the way of the operator.

Mold release layer Working principle

It is not possible to verify the proper application of the release agent. Also, an elastomer layer requires of calibration to correct the collected data. Either way, an operative protocol must be defined, which must ensure proper functioning of the equipment.

Add a glass window to the device

Functionality

Image processing gives feedback of the process and voids formation information, but high-processing computing power is required to process the information online.

Working principle

The glass prevents the addition of heaters or other sensor types in the upper side of the mold. It also prevents obtaining a uniform unidirectional heat flow, which is generally achieved by heating the upper and lower part of the mold.

Reduce the human intervention

Ergonomics

A computer air-assisted increase the success rate and reduce the operator decision-making during the molding process. The device must be operated by one operator.

Safety

The operator will be safe behind the automation, but all the risk will be translated to the device. In case of failure the device could finish with polymerized resin.

In that case critical parts must be protected for this eventuality.

Table 3.1 (continued): Mechatronic device design challenges Challenge Evaluation

Criteria Relevance to the project Easy to unmold the

part Assembly

Glass window, heaters, cooling, sensors, isolation and wires are all in the mold. At the same time the design must deal with the polymerized ports to easy unmolding the part.

Prevent resin leaks

Assembly

Seals, quick connectors, O-ring and mechanical seals are inspected as part of the setup. Even after visual inspection and proper assembly, it is difficult to detect failing seals unless all are brand new.

Cost

Valves, quick connectors, seals could be cleaned and reuse. But knowing the number of tests until failure could not be possible. Instead is easier changing them.

This increases the molding costs.

Functionality Proper sealing is required to obtain accurate and repetitive results.

Resin bleeding Production Allow the resin to flow after filling the mold increases the amount of resin used in one injection.

Environmental Increases the material waste.

Injection Working

principle

It is necessary to know the resin viscosity behavior and the fiber impregnation before attempt to inject.

Properties tested in a separated test.

3.4 Conclusion

After careful examination of the manufacturing of PMCMs, we conclude that the idea of the catch pot provides a protection layer to the device that should be implemented. Also, having a glass window in the mold and image processing from the video recording will provide the device with vital information about the process. It will allow and help the operator to observe the mold filling or fiber impregnation, understand and measure the voids formation during cure [102] and detect gelation time. We acknowledge that all tests were made with resin only, but it is expected that the fibers will be confined to the mold cavity. Two potential problems could arise; first, the fiber content, wettability and compaction could increase the injection pressure to a value higher than the available. And second, if the mold is filled with short fiber content, it could be pulled out during the injection.

In respect to the device design we found that having a critical view of the manufacturing process allowed us to separate the limitations of the RTM scale device from the manufacturing challenges.

Considering these potential problems during the design stage will improve the device functionality.

In a broader context, we were able to reach the first objective (OB1) of identifying the laboratory equipment limitations and manufacturing challenges. In particular, the conclusions presented during the literature review are now supported by the experimentation in the RTM laboratory scale device. We face the manufacturing challenges and problems by ourselves which give us the experience and information to complete this first objective. Moreover, we identify the challenges and organized them in terms of quality, functionality, maintenance, etc. All the information compiled in Table 2.1 and Table 3.1 will serve as a source to obtain the device design requirements.

CHAPTER 4 INTEGRATED MECHATRONICS DESIGN